How Timing Played a Role in the Kepler Discovery of a “Tatooine” Planet Orbiting Two Stars

It’s one of the most iconic images from the Star Wars franchise. Luke Skywalker staring out over the arid landscape of Tatooine as two suns set.

A team of astronomers have found a planet whose sky would look a lot like the image above. If it wasn’t a gas giant. Kepler-453b is the 10th “circumbinary” planet discovered by NASA’s Kepler Mission. A circumbinary planet is a planet that orbits two stars instead of one.

Kepler-453b is also located within the “habitable zone,” the area where liquid water and life could exist. We know how the habitable zone works around single-star systems like our own. But, what about binary systems?

The image below is of a different planetary system; Kepler-47. But, it shows you how the habitable zone appears in binary systems. In Kepler 47’s case, the habitable zone is ring-shaped and centers around the larger star. The zone fluctuates as the two stars orbit each other.

Image credit: NASA

Timing was the most important role in the discovery of Kepler-453b.

“If we had observed this planet earlier or later than we did, we would have seen nothing and assumed there was no planet there,”said Stephen Kane, an assistant professor of physics and astronomy at San Francisco State University.

“That suggests that there are a lot more of these kinds of planets than we are thinking, and we’re just looking at the wrong time,” added Kane, who was a member of the team that made the discovery.

To understand why timing was so important, you need to understand how astronomers discover exoplanets. Astronomers look for transits, or when the planet passes between its host star and Earth. Small changes in the light are the telltale sign of an exoplanet.

But, binary systems are tricky. Gravitational pull from two stars makes the orbit of Kepler-453b inconsistent. Kane compares it to “a spinning top.” Kepler-453b’s transits are visible by astronomers just 9 percent of the time. If Kane and his fellow astronomers didn’t spot Kepler 453b when they did, they might never have seen it. Based on their calculations, the planet won’t be detectable again for another 50 years.

“It’s a good reminder that there’s always a value in checking again,” said Kane.

Astronomers observed Kepler-453b blocking 0.5 percent of its host stars’ light during the transit. By determining how much light is blocked, astronomers can figure out the planet’s size. In this case, the planet has a radius 6.2 times that of Earth. That puts in the gas giant category and unable to have life despite its position in the habitable zone.

Kane points out that the planet could have rocky moons that could potentially support life.

Kepler-453b is the third of the ten circumbinary planets discovered to lie within the habitable zone of a binary system.